1. Field of the Invention
The present invention relates to telecommunications; more particularly, to transmit power control.
2. Description of the Related Art
FIG. 1 illustrates base station 10 and mobile terminal 12 of a CDMA (Code Division Multiple Access) communication system. Base station 10 and mobile terminal 12 communicate in the direction from base station 10 to mobile terminal 12 over forward or downlink channels 14. Mobile terminal 12 communicates with base station 10 using reverse or uplink channels 16. Forward link channels 14 may include separate channels for voice, data, a pilot signal, a dedicated control channel (DCCH) and other signals that provide control or overhead information. In a similar fashion, reverse channel 16 may include a voice channel, a data channel, a pilot channel and a dedicated control channel (DCCH). Depending on the type of communication going on, the voice channels may be omitted and data channels may be used, or data channels may be omitted and voice channels may used, or both voice and data channels may be used.
The amount of power used to transmit in the forward direction is adjusted to account for signal losses that may occur due to environmental conditions and/or signal fading that is a result of multiple paths between the base station and mobile. Mobile 12 monitors characteristics of the forward link such as overall power received or error rates. Based on the received power or the error rates, the mobile instructs base station 10 to either increase or decrease the amount of power used for transmission over the forward data and/or voice channels. For example, if the error rates are increasing or if they cross a threshold that was established when communications were initiated, the mobile instructs the base station to increase the power used for transmitting the forward voice and/or data channels. Similarly, if the mobile detects extremely low error rates, the mobile instructs base station 10 to decrease the power used for transmitting the forward voice and data channels. When the base station receives the power control instructions from the mobile, the base station modifies the power used to transmit the forward voice and data channels accordingly,
FIG. 2 illustrates the pilot channel transmitted from mobile 12 to base station 10. The pilot channel is used by the mobile to transmit power control instructions to the base station. The pilot channel is broken into segments or power control groups 20 of 1.25 milliseconds in length. During portion or power control sub-channel 22 of each of the 1.25 milliseconds segments, the pilot signal is replaced by a power control signal, bit or command(s). The power control signal simply tells the base station to increase or decrease the power used to transmit the forward voice channel and forward data channel. The same power control information is used to control both the forward voice and data channels. This is acceptable since both channels are transmitting into the same environment and will suffer the same fluctuations due to the environment or fading. The remaining three-quarters of each 1.25 milliseconds segment is used for transmitting the pilot signal which enables the base station to coherently demodulate signals from the mobile.
FIG. 3 illustrates a mobile that is in a soft handoff condition where signals are received from two base stations. In this situation mobile 12 is receiving voice and data channels and a pilot channel from base station 10 and from base station 50. As was discussed with regard to FIG. 1, mobile 12 monitors the power and/or error rates associated with the voice and data signals and requests base stations 10 and 50 to either increase or decrease the power used to transmit voice and/or data on the forward link. In this situation, the mobile once again transmits power control commands as was described with regard to FIG. 2, and the same power control command is provided to both base station 10 and base station 50.
FIG. 4 illustrates the situation where mobile 12 is in a soft handoff condition receiving communications from base station 10 and base station 50. In this situation, mobile 12 is receiving a voice and data channel from base station 10, but only a voice channel from base station 50. This situation might arise in a soft handoff where base station 50 does not have sufficient resources available to provide both a voice and data channel to mobile 12, and therefore only provides a voice channel. Since this is an asymmetrical situation, the same power control information cannot be used to control both the voice and data forward channels. The voice and data are subjected to different fluctuations, the power control information derived from monitoring the voice cannot be applied to data and vice versa.